The present invention relates to a detection method in which a CDMA-coded data signal is transmitted in the form of a data stream of spread data bursts between a transmitter and a receiver, hierarchical CDMA codes being used for the transmission, as well as a corresponding detection device.
Although applicable in principle to any data transmissions, the present invention and the idea upon which it is based are explained with reference to a cellular CDMA detection system (CDMA=code division multiple access).
In particular, using code multiple access (CDMA code division multiple access), a plurality of data streams can be transmitted simultaneously via a shared frequency band. In this context, the data-stream symbols to be transmitted are converted by so-called spreading codes into signal pieces having the length of the spreading codes. The signal pieces of the data symbols of one instant are summed up, and the summed pieces are sent according to their chronological sequence, as is known from Nachrichtenxc3xcbertragung [Information Transmission] by K. D. Kammeyer, 2nd edition, Information Technology Series, Teubner, Stuttgart, 1996.
The data symbols of the data streams can be recovered from the received signal pieces by despreading using the spreading codes. When transmitting the signals over a plurality of paths (as, for example, in the case of mobile radio communications), it is advantageous to carry out the despreading separately for all paths, and to sum up the despread signals of all paths weighted (with the coefficients of the channel pulse response). A CDMA receiver functioning in this manner is called a rake receiver.
The multi-path transmission causes mutual interferences of the transmitted signals at the receiver. In contrast to the rake receiver, these interferences can be eliminated, for example, by joint detection (JD=joint detection) in the receiver, as is known from xe2x80x9cZero Forcing and Minimum Mean-Square-Error Equalization for Multiuser Detection in Code-Division Multiple Access Channelsxe2x80x9d by A. Klein, G. K. Kaleh and P. W. Baier, IEEE Trans. Vehic. Tech., Vol. 45 (1996), 276-287.
CDMA codes having a large spreading factor can be constructed from two codes having a smaller spreading factor by Kronecker multiplication. A quantity of CDMA codes thus constructed is called hierarchical. The OVSF codes (OVSF=Orthogonal Variable Spreading Factor) used in the UMTS (UMTS=Universal Mobile Telephone System) are hierarchically constructed codes (see Concept Group Alpha: xe2x80x9cEVALUATION DOCUMENT (DRAFT 1.0), Part 1xe2x80x9d, ETSI Tdoc SMG2 270/97).
It is furthermore known that initially the received CDMA signals having a smaller than necessary spreading factor are detected, and are subsequently despread with the codes used for constructing the hierarchical code.
Large spreading factors permit the use of a great number of codes. The detection of a great number of codes is very costly.
An idea underlying the present invention is that the codes are detected stepwise. First, codes having a small spreading factor are detected. The results thus obtained are then used in detecting codes having a larger spreading factor. Consequently, only a part of the total code tree has to be detected, since only the despread signals of existing codes are passed on to the next step. The existence is expediently determined by the fact that the power (performance) of the codes exceeds a specific threshold.
The detection method and device of the present invention offer the special advantage that they permit detection of hierarchical codes with lower expenditure.
According to one preferred further development, in the first step, the master (parent) code c(1,1) having the spreading factor 1 is detected.
In another preferred embodiment, only a part of the total code tree is detected.
According to a further preferred refinement, OVSF codes are detected.
In another preferred development, the respective preceding step is taken into account in the manner that the powers of the CDMA signals already despread are determined, and only the already despread CDMA signals which exceed a predefined power threshold are passed on to the next step.